1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for adaptively changing a pilot pattern according to a link in an Orthogonal Frequency Division Multiplexing (OFDM) communication system.
2. Description of the Related Art
Although there are many wireless communication technologies proposed as candidates for high-speed mobile communications, OFDM is considered the most prominent future-generation wireless communication technology. It is expected that OFDM will be adopted for most wireless communication applications by the year 2010. OFDM has been adopted as a standard for an Institute of Electrical and Electronics Engineers (IEEE) 802.16 Wireless Metropolitan Area Network (WMAN) categorized into the 3.5th Generation (3.5G) technology.
In a conventional OFDM communication system, a transmitter simultaneously sends pilot subcarrier signals to a receiver with transmission of data subcarrier signals. The receiver performs synchronization acquisition, channel estimation, and Base Station (BS) identification using the pilot subcarrier signals. A transmission rule for sending the pilot subcarrier signals is known as “pilot pattern”.
The pilot pattern is determined in consideration of coherence bandwidth and coherence time. The coherence bandwidth is the maximum bandwidth over which a channel is relatively constant or non-distorting in the frequency domain, and the coherence time is the maximum time for which the channel is relatively constant in the time domain. Since the channel can be assumed to be constant over the coherence bandwidth for the coherence time, one pilot signal suffices for synchronization acquisition, channel estimation and BS identification.
Conventionally, a fixed pilot pattern is used and existing adaptive pilot pattern techniques focus on optimization of pilot power and throughput. A technique for adaptively changing a pilot pattern according to a link status (e.g. coherence bandwidth and coherence time) is yet to be developed.
Radio channels are said to be a wide range of random channels and it is difficult to always ensure optimum performance over these random channels with the conventional fixed pilot pattern. Assuming that a pilot pattern is created using the same number of pilots, a layout of pilot subcarriers affects performance according to the link status, which in turn directly influences channel estimation performance. That is, the channel estimation performance may increase according to the pilot pattern. Since the channel estimation performance has an influence on Bit Error Rate (BER), it is important to select a pilot pattern that minimizes channel estimation errors.